Analog front ends (AFES) play a key role in a number of applications which utilize an interface between a phone line and a digital processor. Typically any one of the digital subscriber line (xDSL) modems available utilize analog front end (AFE) circuits to connect a digital subscriber line (DSL, or more generally xDSL) circuits to an analog telephone line. An xDSL signal is one of the many available digital subscriber line signals that are applicable to the disclosure presented below. In transmitting a DSL signal from the modem onto the telephone line, digital to analog converter (DAC) circuits disposed in digital subscriber line (DSL) modem's interface to an analog telephone line carrying analog signals. In a DSL modem receiving an analog signal from a telephone line, analog to digital converter (ADC) circuits disposed in digital subscriber line (DSL) modem's interface to an analog telephone line carrying analog signals.
In typical DSL modem circuits DAC outputs drive “balanced” analog phone lines “differentially”. Also, analog signals received from the analog telephone line, by the modem are received “differentially” by “balanced” circuits. Typically circuits are “single ended” rather than “balanced” due to the added complexity of adding an extra parts utilized in balanced circuits. Balanced circuits utilize two signal carrying conductors and a ground conductor. Single ended circuits utilize a single signal carrying conductor and a ground conductor. Typically a balanced signal is transformed into a single ended signal after reception and initial processing in which the signal strength is typically improved.
Voice band modems (such as those built under the v.90 standard described in the International Telecommunications Union ITU v.90 standard for providing digital communications between computers via twisted pair telephone lines are well known. Voice band modems are commonly used to provide Internet access by facilitating digital communications between personal computers and Internet Service Providers (ISPs).
Due to the increasingly large quantity of digital data being communicated via twisted pair telephone lines, the maximum bit rate associated with voice band modems is frequently considered inadequate. The comparatively slow speed of voice band modems is a severe limitation when transferring large binary files such as images, film clips, audio, large data files and the like.
The increasing popularity of such communication services as video on demand (pay-per-view), realtime video teleconferencing and high speed Internet access has further increased the need for higher data rates over twisted pair telephone lines. Digital subscriber line (DSL) provides a way of facilitating digital communications over twisted pair telephone lines at data rates in excess of 1.5 Mbps, so as to facilitate such desirable services.
DSL utilizes an advanced modulation scheme known as quadrature amplitude modulation (QAM), wherein a combination of amplitude and phase modulation is used to encode digital information for transmission over twisted pair copper telephone lines. Although QAM provides a substantial increase in bit rate, as compared with earlier modulation schemes such as those which are utilized in contemporary voice band modems, it is still desirable to optimize the bit rate provided by QAM, so as to provide digital communications at the highest possible speed while maintaining the desired quality of service.
One problem which inhibits optimization of the bit rate in DSL installations is radio frequency ingress (RFI). RFI occurs when the twisted pair copper wires of a DSL installation function as a radio antenna at the frequencies upon which the DSL transceivers communicate. Conventional DSL transceivers include balanced analog front ends for common mode rejection of RFI. However, balanced front ends often increase complexity and cost. Balanced AFEs are thought to provide the cleanest means for signal transmission between the telephone line and the DACs and ADCs of the DSL modem. Balanced AFEs typically keep a noise floor associated with a transmitted signal down to a minimum. Balanced AFEs typically use double the number of parts a single ended AFE would consume, but tend to minimize undesirable noise present in the noise floor. Additionally, active parts typically utilized in an a balanced AFE to improve signal quality typically consume more power than their single ended counterparts. Accordingly, an unbalanced analog front end with RFI rejection is desirable.